The spine is comprised of twenty-four vertebrae that are stacked one upon the other to form the spinal column. The spine provides strength and support to allow the body to stand and to provide flexibility and motion. Each vertebra includes an opening through which the spinal cord passes enabling the spine to protect the spinal cord. The spinal cord includes thirty-one pairs of nerve roots that branch from either side of the spinal cord, extending through spaces between the vertebrae known as the neural foramen.
Between each pair of vertebrae is an intervertebral disc. The disc is composed of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral endplates. The disc serves several purposes, including absorbing shock, relieving friction, and handling pressure exerted between the superior and inferior vertebral bodies associated with the disc. The disc also absorbs stress between the vertebral bodies, which stress would otherwise lead to degeneration or fracture of the vertebral bodies.
Disorders of the spine are some of the costliest and most debilitating health problems facing the populations of the United States and the rest of the world, costing billions of dollars each year. Moreover, as those populations continue to age, the incidence of spinal disorders will continue to grow. Typical disorders include those caused by disease, trauma, genetic disorders, or other causes.
The state of the art includes a number of treatment options. Medicinal treatments, exercise, and physical therapy are typical conservative treatment options. Less conservative treatment options include surgical intervention, including microdiscectomy, kyphoplasty, laminectomy, dynamic stabilization, disc arthroplasty, and spinal fusion. Traditionally, these treatment options have been utilized in isolation, rather than in combination, and the most conservative of the treatment options utilized to provide a desired result.
U.S. Provisional Patent Application Ser. No. 60/713,671, entitled “Prosthetic Intervertebral Discs,” (“the '671 application”), was filed Sep. 1, 2005, and is assigned to Spinal Kinetics, Inc., the assignee of the present application. The '671 application describes, inter alia, a treatment option that combines a prosthetic intervertebral disc with a dynamic stabilization system. The '671 application is incorporated by reference herein in its entirety.
In 1992, Panjabi introduced a model of a dynamic spinal stabilization system that describes the interaction between components providing stability in the spine. This model defined spinal instability in terms of a region of laxity around the neutral resting position of a spinal segment, identified as the “neutral zone.” Panjabi, M M., “The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis.” J Spinal Disord 5 (4): 390-397, 1992b. There is some evidence that the neutral zone increases as a result of intervertebral disc degeneration, spinal injury, and spinal fixation. Id. Panjabi has subsequently described dynamic stabilization systems that provide increased mechanical support while the spine is in the neutral zone and decreased support as the spine moves away from the neutral zone. See United States Published Patent Application No. 2004/0236329, published Nov. 25, 2004, which is hereby incorporated by reference herein.